This invention relates to an anticorrosive treatment for galvanized or zinc-coated steels, and more particularly to a method of anticorrosive treatment which protects zinc coatings on the surfaces of steel materials against white rust.
It is the general practice in the art to provide a zinc coating on the surface of steel by galvanization or other means for protection against rust or corrosion. However, the surface of the zinc coating is susceptible to white rust when exposed to the atmosphere due to reactions with moisture or carbon dioxide in the atmosphere.
In order to prevent the formation of white rust, the zinc-coated steel may be further treated with a chromate. Although the chromate treatment has the advantages of excellent anticorrosive properties, simplicity and low cost, the use of chromate is restricted by environmental polution regulations and causes problems such as toxicity to workers of chromate spattered during the treating process, difficult disposal of the chrome sludge after the treatment of the spent liquor, possible chrome exudation on products after the treatment with chromate, inferior adhesion of paint, and the like.
In another process, a treatment with a phosphate is employed to improve the corrosion resistance of galvanized steel and paint adhesion thereto, but it is far inferior to the chromate treatment in imparting corrosion resistance.
For these reasons, various techniques have recently been proposed and applied for preventing formation of white rust on zinc-coated steel materials using a pollution-free substance instead of chromate, for example, using inorganic compounds, organic compounds, organic macromolecular compounds (mainly resins) or combinations thereof, or using immersion, coating or electrolysis. The following are some examples of these methods.
In Japanese Patent Publication No. 6846/71, there is described a method for forming an anticorrosive coating by immersing a zinc-coated steel in a treating solution consisting mainly of an aqueous solution of molybdate of a concentration less than 0.5 M and an aqueous solution of 0.05-45 wt. % of a water-soluble organic compound or an organic macromolecular compound, followed by drying by heating.
Japanese Patent Publication No. 2419/76 discloses a method for forming an anticorrosive coating by immersing a zinc-coated steel in an aqueous solution 1/40-1/50 M in magnesium molybdate or calcium molybdate.
Further, Japanese Laid-open Patent Specification No. 14141/76 discloses a method for forming an anticorrosive coating by immersing a zinc-coated steel in an aqueous solution of ammonium molybdate containing ammonium sulfate.
The methods of the above-mentioned publications are more or less effective for preventing the production of white rust on the zinc coated steel materials but require a complicated process for the preparation of the treating liquid or a long processing time.
Hence a need has continued to exist for a method of protecting galvanized steel from white rust which is simple and free from the pollution problems associated with chromate solutions.